An electric field sensor that utilizes an EO effect inputs an optical beam to an EO crystal to which an alternate current electric field is applied, makes a polarizing beam splitter (hereinafter referred to as a “PBS”) split the light output from the EO crystal into an S polarized light and a P polarized light, and makes two photo detectors (hereinafter referred to as “PDs”) independently detect the respective polarized lights. A differential amplifier detects a difference between intensities of the polarized lights.
Immediately before the optical beam is incident to the PBS, the optical beam is desirably a circularly polarized light. Main advantages of the circularly polarized light are listed below.    (1) Intensity modulation of the optical beam becomes a maximum, which contributes to a highly sensitive detection of a differential signal. Based on a detection of the differential signal, the amplitude of an output signal from the PD can be amplified to a double.    (2) Based on a detection of the differential signal, intensity noise of the optical beam can be reduced, which contributes to a highly sensitive detection of the differential signal.    (3) Based on a detection of the differential signal, a direct current component of the signal can be offset, which contributes to a reduction in the load of a signal processing circuit.
FIG. 1 is an explanatory diagram of the operation of a conventional electric field sensor.
An optical beam 3 emitted from a light source 1 passes through a quarter wave plate (hereinafter referred to as a “QWP”) 5 and an EO crystal 7, and is incident to a PBS 9. The QWP 5 adjusts the polarization state of the optical beam 3 such that the polarized light becomes a circularly polarized light immediately before the optical beam is incident to the PBS 9. An alternate current electric field corresponding to an alternate current signal under test 15 is applied to the EO crystal 7 via a signal electrode 11 and a ground electrode 13. The optical beam 3 is modulated in a polarizing manner within the EO crystal 7 according to the electric field. The PBS 9 splits the modulated light into S and P polarized light components. In this case, each polarized light component is converted into an intensity-modulated light. The intensity-modulated S and P polarized light components change in mutually reversed phases, and PDs 17 and 19 receive the output lights. A differential amplifier 21 detects a differential signal, thereby making it possible to obtain an output signal 22 in higher sensitivity.
FIGS. 2(a) to 2(f) are diagrams showing a relationship between a polarization state of an optical beam incident to the PBS 9 and an electric signal corresponding to the polarization state.
As shown in FIG. 2(a), when the polarization state of the optical beam that is incident to the PBS 9 is kept as a circularly polarized light, the intensities of the S and the P polarized light components obtained by splitting the optical beam by the PBS 9 are equal.
As shown in FIG. 2(b), when the polarization state of the optical beam that is incident to the PBS 9 is kept as a circularly polarized light, the electric signals output from the PDs 17 and 19 become signals corresponding to signals under test (in this case, sinusoidal waves) using 0.5 Vmax (where Vmax is an output voltage of the PD corresponding to a total amount of light) as a reference value. These signals mutually change in reversed phases. Maximum amplitude of each signal is set as A.
As shown in FIG. 2(c), when the polarization state of the optical beam that is incident to the PBS 9 is kept as a circularly polarized light, maximum amplitude of an output signal from the differential amplifier 21 becomes 2A, and a direct current component is offset. In this case, optical intensity noise included in the output signals from the PDs 17 and 19 is reduced substantially.
The above technique is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2003-98205 and 2001-324525.
However, as shown in FIG. 2(d), when the optical beam that is incident to the PBS 9 is not kept as a circularly polarized light but is in an elliptically polarized light, unbalance occurs in the intensities of the S and the P polarized light components obtained by splitting the optical beam by the PBS 9. This is mainly due to a temperature change.
As shown in FIG. 2(e), when the optical beam that is incident to the PBS 9 is in an elliptically polarized light, unbalance occurs in the direct current component of electric signals output from the PDs 17 and 19 (Vs≠Vp, and Vs+Vp=Vmax). As compared with the state shown in FIG. 2(b), a signal amplitude rA becomes smaller (rA, 0≦r<1).
As shown in FIG. 2(f), when the optical beam that is incident to the PBS 9 is an elliptically polarized light, a direct current component (Vp−Vs) remains in the electric signal output from the differential amplifier 21. The amplitude becomes 2rA, which is smaller than that shown in FIG. 2(c). In this case, the optical intensity noise included in the output signals from the PDs 17 and 19 cannot be reduced sufficiently.
When there is a temperature change, it is difficult for the optical beam immediately before being incident to the PBS 9 to maintain always as a circularly polarized light as the polarization state.
As explained above, because the optical beam changes to an elliptically polarized light in the electric field sensor, the following disadvantages arise.    (1) Since the intensity modulation of the optical beam decreases, the sensitivity of the sensor decreases.    (2) Unbalance occurs in the average intensity of the S and the P polarized light components. Laser intensity noise cannot be sufficiently reduced by detecting a differential signal. Therefore, the sensitivity as the sensor decreases.    (3) A direct current component of a signal cannot be sufficiently decreased by detecting a differential signal.
An undesirable reflected light in the above electric field sensor is explained next.
FIG. 3 is a diagram showing a configuration of an electric field sensor similar to that shown in FIG. 1, particularly focusing on the reflected light based on the polarization state. In FIG. 3, the electrodes 11 and 13 are omitted.
In an electric field sensor 101, a P polarized light 108 is irradiated from the laser light source 1. The P polarized light 108 is converted into a circularly polarized light 109 by the QWP 5.
The EO crystal 7 modulates the circularly polarized light 109 in a polarizing manner. The PBS 9 splits the modulated light into a P polarized light (component) 110 and an S polarized light (component) 111. The PD 19 receives the P polarized light 110, and converts this light into an electric signal. On the other hand, the PD 17 receives the S polarized light 111, and converts this light into an electric signal. The differential amplifier 22 or the like shown in FIG. 1 differentially amplifies these electric signals. An electric field is measured based on the differentially amplified result.
In the following explanations, the EO crystal 7, the PBS 9, and the PDs 17 and 19 are collectively called a reflection element 107, for the sake of convenience. Details of the reflection element 107 are described later.
FIG. 4 is a diagram for explaining the problems of the above electric field sensor 101 to be solved.
As described above, a circularly polarized light is incident to the EO crystal 7 in the electric field sensor 101. Each device including the EO crystal 7 constituting the reflection element reflects the incident light. The reflected light becomes a reflection return light 112, which is converted into an S polarized light 113 by the QWP 5. When the reflection return light (S polarized light) 113 is reversely incident to the laser light source 1, this can affect the measurement precision of the electric field sensor 101.
A reflected light can easily be generated on a light receiving surface of the PD, not to mention on the end surface of the EO crystal. When the reflected light on the light receiving surface of the PD is reversely incident to the laser light source 1, this can also affect the measurement precision of the electric field sensor 101.
The above electric field sensor is also applied to a detector of an electric signal that is transmitted through a human body as shown in FIG. 5, and a detector of an electric signal in a device under test (hereinafter referred to as a “DUT”) as shown in FIG. 6 (refer to Japanese Patent Application Laid-Open No. 2000-171488).
As shown in FIG. 5, a receiving electrode RP is in contact with a measured point of a human body 100. When an electric signal is input to the receiving electrode RP from a signal source Sin via a transmitting electrode SP and the human body 100, a signal electrode 11 within the electric field sensor connected to the receiving electrode RP with a lead wire has the same potential as that of the measured point.
As shown in FIG. 6, a metal needle MN is in contact with a measured point of a DUT 201. When an electric signal is input to the metal needle MN from the signal source Sin via the DUT 201, the signal electrode 11 within the electric field sensor connected to the metal needle MN with the lead wire LD has the same potential as that of the measured point.
The subsequent operations are the same in FIG. 5 and in FIG. 6. First, an electric field is generated between the signal electrode 11 and the ground. In this case, a part of lines of electric force passes through the EO crystal 7, thereby generating an electric field within the EO crystal 7, as shown in FIG. 5 and FIG. 6.
When the electric field is generated within the EO crystal 7, a birefringent index changes within the EO crystal 7 according to the electric field. When a circularly polarized light is incident directly from the light source 1 or via the QWP 5 to the EO crystal 7 in which a birefringent index changes, an elliptically polarized light is output from the EO crystal 7. The elliptically polarized light is reflected from two mirrors 14a and 14b, and the reflected light is incident to the PBS 9. The PBS 9 splits the light into two linearly polarized lights (S and P polarized lights). The two PDs 17 and 19 detect the S and the P polarized lights. Electric signals proportional to the respective intensities of the polarized lights are input to the differential amplifier 21. An electric signal output from the differential amplifier 21 is proportional to the amplitude of the electric field within the EO crystal 7. Therefore, by measuring the electric signal output from the differential amplifier 21, the amplitude of the electric field within the EO crystal 7 can be detected. The amplitude of the electric field within the EO crystal 7 is proportional to the potential at the measured point of the human body 100 or the DUT 201. Therefore, by detecting the electric signal output from the differential amplifier 21, the potential at the measured point can be detected.
However, according to the conventional electric field sensor, as shown in FIG. 5 and FIG. 6, only a small part of the lines of electric force generated from the signal electrode 11 passes through the EO crystal 7. Therefore, the amplitude of the electric field within the EO crystal 7 is small. Consequently, the polarization state of the optical beam from the light source 1 cannot be modulated sufficiently. As a result, high sensitivity as the electric field sensor cannot be obtained.